1. Technical Field
The invention generally relates to producing crude oil or natural gas from a well drilled into a subterranean formation. More particularly, the invention is directed to improved treatment fluids and methods that are capable of forming crosslinked gels in subterranean formations. A particular application of the invention is for conformance control. Production of unwanted water from a hydrocarbon producing well can be a limiting factor in the productive life of a well.
2. Background Art
Oil or gas is naturally occurring in certain subterranean formations. A subterranean formation having sufficient porosity and permeability to store and transmit fluids is referred to as a reservoir. A subterranean formation that is a reservoir for oil or gas may be located under land or under a seabed offshore. Oil or gas reservoirs are typically located in the range of a few hundred feet (shallow reservoirs) to a few tens of thousands of feet (ultra-deep reservoirs) below the ground or seabed.
In order to produce oil or gas, a wellbore is drilled into a subterranean formation that is an oil or gas reservoir. A wellbore can include an openhole or uncased portion. A wellbore can have vertical and horizontal portions, and it can be straight, curved, or branched.
Various types of treatments are commonly performed on wells or subterranean formations penetrated by wells. For example, stimulation is a type of treatment performed on a subterranean formation to restore or enhance the productivity of oil or gas from the subterranean formation. Stimulation treatments fall into two main groups: hydraulic fracturing and matrix treatments. Fracturing treatments are performed above the fracture pressure of a subterranean formation to create or extend a fracture in the rock. The fracture is propped open with sand or other proppant to provide a highly permeable flow path between the formation and the wellbore. Matrix treatments are performed below the fracture pressure of a subterranean formation. Matrix treatments can include, for example, treatments to consolidate a matrix of unconsolidated rock particles so that less particulate is produced with the produced hydrocarbon or to alter the permeability of the matrix of a subterranean formation to improve fluid flow through the formation.
When oil or gas is produced from subterranean formations, water often accompanies the produced oil or gas. The source of the water can be a water producing zone communicating with the oil or gas producing formation through a fracture, high-permeability streak, high-permeability zone, and the like, or it can be caused by a variety of other occurrences which are well known to those skilled in the art, such as water coning, water cresting, bottom water, lateral channeling, channeling at the wellbore, etc.
In addition, the source of the water can be the result of waterflood techniques, which is a type of secondary recovery to improve production of oil. Secondary recovery is the second stage of hydrocarbon production during which an external fluid such as water, gas, or alternating both fluids is injected into the reservoir through one or more injection wells penetrating a subterranean formation that has fluid communication with a production well. The purpose of secondary recovery is to maintain reservoir pressure and to displace hydrocarbons toward the wellbore of a production well. In waterflooding, water is injected into a reservoir to displace residual oil. The water from injection wells sweeps the displaced oil toward a production well. Potential problems associated with waterflood techniques include inefficient recovery due to variable permeability and other conditions affecting fluid transport within the reservoir. Early water breakthrough to the production well may cause production and surface processing problems.
Conformance control is a type of well treatment directed to improve the injection or production profile of a well. Conformance control is sometimes referred to as profile modification. Conformance control encompasses procedures that enhance recovery efficiency, such as by reducing the proportion of water produced with the oil or gas. Problems of high water production caused by permeability variations in a subterranean formation have been corrected, for example, by reducing the permeability of a portion of the subterranean formation having high permeability and low oil or gas content.
There are at least two types of methods for reducing the permeability of a portion of a subterranean formation. One method involves the injection of a polymer that is capable of being crosslinked to form a gel within the matrix of the subterranean formation. The gel physically blocks fluid flow through the portion of the formation in which the gel has been placed, directing all fluid flow around the portion of the formation or inducing the production from the non-drained portions. This method is sometimes referred to as permeability blocking. As a result of this kind of treatment, fluid flow is directed through other portions of the subterranean formation having lower permeability. The polymer compositions for use in this method are sometimes referred to as crosslinkable polymer compositions.
Another method for reducing the permeability of a subterranean formation involves the injection of a chemical that attaches to adsorption sites on the rock surfaces within the matrix of the subterranean formation. The attached chemical is adapted to reduce the water permeability through the formation without substantially reducing the hydrocarbon permeability. These chemicals are sometimes referred to as relative permeability modifiers.
Crosslinkable polymer compositions have included, for example, water-soluble polymers including copolymers of acrylamide and acrylic acid crosslinked with chromium or other transition metal ions. In accordance with an early technique, an aqueous solution of one or more of the polymers or copolymers mixed with a crosslinking metal ion is injected into the subterranean formation and allowed to cross-link therein. However, it has heretofore been found that the metal cross-linked gels formed have often been ineffective at high temperatures, i.e., at temperatures above about 180° F. (82° C.) because of the instability of the crosslinker or polymer. This has resulted in uncontrolled crosslinking rates (too rapid), crosslinker precipitation, polymer degradation, or inefficient solution propagation through the rock matrix. In attempts to correct these problems, the crosslinking metal ion has been coordinated with a ligand such as acetate or propionate to slow the reaction of the metal ion with the polymer. While this and other techniques have been utilized successfully, the use of some metal ions, e.g., chromium, has adverse environmental effects, and the metal ion used can be adsorbed by formation materials whereby it is prevented from functioning to crosslink the polymer.
U.S. Pat. No. 4,773,481 to Allison et al. entitled “Reducing Permeability of Highly Permeable Zones in Underground Formations,” issued on Sep. 27, 1988, which is incorporated herein by reference in its entirety, describes a process for reducing the permeability of a subterranean formation by the cross-linking of water-soluble polymers of polyalkylene imines and polyalkylenepolyamines with certain polymers which are anionic or hydrolyzable to form anionic polymers. Examples of the anionic polymers are polyacrylamide and alkylpolyacrylamides, copolymers of polyacrylamide and alkylpolyacrylamides with ethylene, propylene and styrene, polymaleic anhydride and polymethylacrylate, and hydrolysis products thereof. As described in the patent, when the water-soluble polymer and the anionic polymer are mixed, a viscous gel is quickly formed. In use, a solution of the water-soluble polymer is pumped into the subterranean formation first, followed by water to displace the water-soluble polymer from the wellbore to thereby prevent premature gelling upon introduction of the anionic polymer. Thereafter, the anionic polymer is pumped into the formation. This three-step procedure has a number of disadvantages in practice and is costly to perform, but it is necessary because the water-soluble polyalkylene imine or polyalkylenepolyamine reacts very quickly with the anionic polymer and cannot be premixed without premature gelation.
U.S. Pat. No. 5,836,392 having named inventor Phillip Lance Urlwin-Smith, entitled “Oil And Gas Field Chemicals,” issued on Nov. 17, 1998, and assigned of record to Halliburton Energy Services, Inc., which is incorporated herein by reference in its entirety, discloses a method for conformance control of a reservoir comprising injecting into a zone of the reservoir an aqueous solution of a co-polymer comprising at least one ethylenically unsaturated polar monomer and at least one copolymerizable ethylenically unsaturated ester formed from a hydroxy compound of the formula ROH wherein R is a selected alkyl group, alkenyl group, cycloalkyl group, aryl group or such groups substituted with from 1 to 3 hydroxyl, ether or thio ether groups or a heterocyclic or selected heterocyclic alkylene group and at least one heteroatom selected from oxygen, nitrogen and sulfur and a selected alkenoic or aralkenoic carboxylic acid or sulfonic or phosphoric acid together with a crosslinking agent comprising a multi-valent metal ion capable of crosslinking an acrylic acid polymer to form a viscous gel. The injected fluid is flowed through at least a portion of a high permeability region within said zone wherein it is heated to an elevated temperature whereupon crosslinking of the polymers occurs to form a substantially non-flowable gel within said high permeability region. The crosslinking of the injected fluid to form the non-flowable gel within the formation reduces the permeability of said region in said zone.
U.S. Pat. No. 6,192,986 to Phillip Lance Urlwin-Smith, entitled “Blocking Composition For Use In Subterranean Formation,” issued on Feb. 27, 2001, and assigned of record to Halliburton Energy Services, Inc., which is incorporated herein by reference in its entirety, describes a way of avoiding the use of metal ion cross-linking agents and of controlling the gelling rate of polymers whereby premixes of polymer and a gelling agent can be made and safely injected into a downhole formation without serious risk of premature gelation. The composition comprises a water-soluble copolymer comprising (i) at least one non-acidic ethylenically unsaturated polar monomer and (ii) at least one polymerizable ethylenically unsaturated ester; and (iii) at least one organic gelling agent, characterized in that the gelling agent is a polyalkyleneimine, polyfunctional aliphatic amine, an aralkylamine, or a heteroaralkylamine. The gelling agents are free from metal ions, and are preferably water-soluble polymers capable of cross-linking the copolymers. Among the preferred water-soluble polymers for use as gelling agents are polyalkyleneimines, polyalkylenepolyamines, and mixtures thereof. Additional details concerning these polymers and their preparation are disclosed in U.S. Pat. No. 3,491,049, which is also incorporated herein by reference in its entirety. The preferred polyalkylenepolyamines are the polymeric condensates of lower molecular weight polyalkylenepolyamines and a vicinal dihaloalkane. The polyalkyleneimines are best illustrated by polymerized ethyleneimines or propyleneimine. The polyalkylenepolyamines are exemplified by polyethylene and polypropylenepolyamines. Other gelling agents which can be used include water-soluble polyfunctional aliphatic amines, aralkylamines, and heteroaralkylamines optionally containing other hetero atoms. The method of conformance control of a subterranean reservoir comprises: (a) injecting into a formation an aqueous solution of a composition of the invention; (b) allowing the solution to flow through at least one permeable zone in said formation; and (c) allowing the composition to gel. As the solution is pumped downhole and permeates into the zone, it heats up and eventually reaches the downhole temperature after which gelling occurs.
U.S. Pat. No. 6,176,315 to B. R. Reddy, Larry Eoff, Jiten Chatterji, San T. Tran, and Dwyann Dalrymple, entitled “Preventing Flow Through Subterranean Zones,” issued on Jan. 23, 2001, and assigned of record to Halliburton Energy Services, Inc., which is incorporated herein by reference in its entirety, discloses methods of preventing the flow of water or gas or both through a subterranean zone having a high temperature and a depth such that a long pumping time is required to place a sealing composition therein. The methods basically comprise the steps of preparing a polymeric sealing composition comprised of water, a cross-linking agent, and a selected water-soluble polymer, which reacts with the cross-linking agent and forms a sealing gel which is stable for a desired period of time at the temperature of the zone and has a pumping time before gelation in the presence of the cross-linking agent, whereby the composition can be pumped to the depth of the zone and placed therein. Thereafter, the sealing composition is pumped into the zone and allowed to form a sealing gel therein. A “gelation accelerating agent” can be utilized to reduce pumping time before gelation at a given temperature. The gelation accelerating agent can be a pH control compound such as an alkali metal carbonate, bicarbonate or hydroxide, a mineral acid such as hydrochloric acid, an organic acid such as acetic acid, a Lewis acid such as boric acid or other compounds such as ammonium chloride, urea and lactose. Of these, boric acid is preferred. When utilized, boric acid is added to the sealing compositions of this invention in a general amount in the range of from about 0.005% to about 0.1% by weight of the composition.
U.S. Pat. No. 6,196,317 to Mary Anne Hardy, entitled “Method and Composition for Reducing the Permeabilities of Subterranean Zones,” issued on Mar. 6, 2001, and assigned of record to Halliburton Energy Services, Inc., which is incorporated herein by reference in its entirety, describes the steps of introducing an aqueous solution of a chelated organic gelling agent and a copolymer of a non-acidic ethylenically unsaturated polar monomer and an ethylenically unsaturated ester into a subterranean zone, and then allowing the aqueous solution to form a cross-linked gel in the zone. The chelated organic gelling agent is comprised of a water-soluble polyalkylene imine chelated with a metal ion, preferably polyethylene imine chelated with zirconium. The non-acidic ethylenically unsaturated polar monomer in the copolymer is an amide of an unsaturated carboxylic acid, preferably acrylamide, and the ethylenically unsaturated ester in the copolymer is formed of a hydroxyl compound and an ethylenically unsaturated carboxylic acid such as acrylic acid, methacrylic acid and the like. A preferred unsaturated ester is t-butyl acrylate. In a further aspect, instead of utilizing the above-described copolymer which is rapidly cross-linked by the chelated gelling agent once the chelated gelling agent disassociates, the copolymer can be stabilized whereby it does not cross-link as rapidly at high temperatures and also has greater long-term gel strength after being cross-linked by forming it into a terpolymer or a tetrapolymer. That is, instead of a copolymer, the above-described non-acidic ethylenically unsaturated polar monomer, preferably acrylamide, and the ethylenically unsaturated ester, preferably t-butyl acrylate, are reacted with AMPS® (2-acrylamido-2-methylpropane sulfonic acid) and/or N-vinylpyrrolidone to produce a terpolymer, e.g., polyacrylamide/t-butyl acrylate/AMPS® or polyacrylamide/t-butyl acrylate/N-vinylpyrrolidone or a tetrapolymer, e.g., polyacrylamide/t-butyl acrylate/AMPS®/N-vinylpyrrolidone. The most preferred terpolymer is polyacrylamide/t-butyl acrylate/N-vinylpyrrolidone. The compositions for reducing the permeability of a subterranean zone are basically comprised of water, a copolymer of an ethylenically unsaturated polar monomer, and an ethylenically unsaturated ester or a terpolymer or tetrapolymer of the aforesaid polar monomer and ester with AMPS® and/or N-vinylpyrrolidone, and a chelated organic gelling agent.
As an example of a relative permeability modifier, U.S. Pat. No. 6,476,196 to Larry Eoff, Raghava Reddy, and Eldon Dalrypmple, entitled “Methods of Reducing Subterranean Formation Water Permeability,” issued Nov. 5, 2002, and assigned to Halliburton Energy Services, Inc., which is incorporated herein by reference in its entirety, discloses introducing into the formation a water flow resisting chemical which attaches to adsorption sites on surfaces within the porosity of the formation and reduces the water permeability thereof without substantially reducing the hydrocarbon permeability thereof. The water flow resisting chemical is comprised of a polymer of at least one hydrophilic monomer and at least one hydrophobically modified hydrophilic monomer.
U.S. Pat. No. 6,838,417 to Ron C. M. Bouwmeester and Klass A. W. Van Gijtenbeek, entitled “Compositions and Methods Including Formate Brines for Conformance Control,” issued Jan. 4, 2005, and assigned to Halliburton Energy Services, Inc., which is incorporated herein by reference in its entirety, discloses compositions and methods are provided for reducing the permeability of subterranean zones. More particularly, water-soluble polymeric compositions which form crosslinked gels in the zones. In general, the composition comprises (a) at least one water-soluble polymer; (b) at least one organic gelling agent capable of cross-linking the water-soluble polymer; and (c) at least one water-soluble formate. More preferably, the water-soluble polymer is a copolymer of (i) at least one non-acidic ethylenically unsaturated polar monomer, and (ii) at least one polymerizable ethylenically unsaturated ester. The gelling agent is preferably a polyalkyleneimine, polyfunctional aliphatic amine, an aralkylamine, and a heteroaralkylamine. The preferred water-soluble formate is selected from the group consisting of ammonium formate, lithium formate, sodium formate, potassium formate, rubidium formate, cesium formate, and francium formate. Water is used to make an aqueous composition prior to use in a subterranean formation. The methods of this invention for reducing the permeability of a subterranean zone are comprised of the steps of introducing an aqueous composition according to the invention into a subterranean zone, and then allowing the aqueous composition to form a cross-linked gel in the zone. Preferably, the method includes the step of subsequently producing hydrocarbons from the subterranean formation.
U.S. Pat. No. 7,091,160 to Bach Dao et al., entitled “Methods and Compositions for Reducing Subterranean Formation Permeabilities,” issued Aug. 15, 2006, and assigned to Halliburton Energy Services, Inc., which is incorporated herein by reference in its entirety, discloses methods and compositions for reducing the permeabilities of subterranean formations or zones are provided. The methods are comprised of introducing an aqueous composition into the formation or zone comprised of water, a water soluble organic polymer, an organic gelling agent for cross-linking the organic polymer and a gel retarder comprised of a chemical compound (e.g., polysuccinimide or polyaspartic acid) that hydrolyzes or thermolyzes to produce one or more acids in the composition and then allowing the aqueous composition to form a cross-linked gel in the formation or zone.
U.S. Pat. No. 7,128,148 to Larry S. Eoff and Michael J. Szymanski, entitled “Well Treatment Fluid and Methods for Blocking Permeability of a Subterranean Zone,” issued Oct. 31, 2006, and assigned to Halliburton Energy Services, Inc., which is incorporated herein by reference in its entirety, discloses a well treatment fluid for use in a well, the well treatment fluid comprising water, a water-soluble polymer comprising at least one unit of vinyl amine, and an organic compound that is crosslinked with the polymer. It also discloses a method of treating a subterranean formation penetrated by a wellbore, the method comprising the steps of: (a) forming a treatment fluid comprising water, a water-soluble polymer comprising at least one unit of vinyl amine, and an organic compound that is crosslinked with the polymer; and (b) introducing the treatment fluid through the wellbore and into contact with the formation.
U.S. Pat. No. 7,287,587 to B. Raghava Reddy, Larry S. Eoff, Eldon D. Dairymple, and Julio Vasquez, entitled “Crosslinkable Polymer Compositions and Associated Methods,” issued Oct. 30, 2007, and assigned to Halliburton Energy Services, Inc., which is incorporated herein by reference in its entirety, discloses crosslinkable polymer compositions comprising an aqueous fluid; a water-soluble polymer comprising carbonyl groups; an organic crosslinking agent capable of crosslinking the water-soluble polymer comprising carbonyl groups; and a water-soluble carbonate retarder. Methods comprising: providing a crosslinkable polymer composition; introducing the crosslinkable polymer composition into a portion of a subterranean formation; and allowing the crosslinkable polymer composition to form a crosslinked gel in the portion of the subterranean formation.
Halliburton Energy Services, Inc. has employed a crosslinkable polymer system of a copolymer of acrylamide and t-butyl acrylate, where the crosslinking agent is polyethylene imine. These materials are commercially available from Halliburton Energy Services, Inc. as part of the H2Zero™ conformance control service. The H2Zero™ service employs a combination of HZ-10™ polymer and HZ-20™ crosslinker. HZ-10™ polymer is a low molecular weight polymer consisting of polyacrylamide and an acrylate ester. More particularly, HZ-10™ polymer is a co-polymer of acrylamide and t-butyl acrylate (“PAtBA”). The HZ-20™ crosslinker is a polyethyleneimine (which is not chelated). The H2Zero™ service for conformance control includes mixing the HZ-10™ polymer with the HZ-20™ crosslinker and injecting the fluid mixture into a well. The relative amounts of HZ-10™ polymer and HZ-20™ crosslinker to be used in the preparation of H2Zero™ can be adjusted to provide gelling within a specified time frame (within certain limits) based on reaction conditions such as temperature and pH. For example, the amount of HZ-20™ crosslinker necessary for gelling is inversely proportional to temperature wherein higher amounts of HZ-20™ crosslinker are required at lower temperatures to effect formation of a viscous gel. Adjustment of the H2Zero™ conformance control service to provide optimum gelling time (within certain limits) as a function of temperature and/or pH is known to one of ordinary skill in the art.
More particularly, it is well known that the gelation time of the HZ-10™ polymer and HZ-20™ crosslinker decreases with increasing temperature. It is also believed that a pH of equal to or greater than 10 was helpful to increase the gelation time.
Although the above-described water-soluble polymer systems crosslinked with organic crosslinkers are generally believed to be thermally stable, for example, it is believed the crosslinked gel of the H2Zero™ service is stable up to about 400° F. (204° C.). However, the use of the polymer gel system in conformance applications at matrix temperatures close to the gel stability temperature is limited by the inadequately short pump times. When gelling compositions utilizing gelation retarders such as the carbonate salts, as described in U.S. Pat. No. 7,287,587 discussed earlier, are used in field water, rich in divalent ions such as calcium ion and magnesium which contribute to the hardness of water, or sea water divalent and multivalent ions, precipitation of solids, presumably composed of insoluble magnesium and calcium carbonates, and other insoluble salts, are formed upon mixing the components. Formation of such solid precipitates renders injection of fluids into the porosity of formation matrix very difficult or impossible without using high injection pressure with the possibility of such pressures exceeding the fracture pressure of the formation matrix. Thus, there are continuing needs for improved compositions and methods for blocking the permeabilities of subterranean formations or zones using a crosslinkable polymer composition where the crosslinking of the polymer is effectively and simply controlled at high temperatures.